Following a long-term fish research and monitoring project, recent changes to environmental flow management have supported Murray cod spawning along the Gunbower Creek. Located between the townships of Torrumbarry and Koondrook in Victoria, the Gunbower Creek, off the Murray River, is primarily an irrigation supply channel. Since 2005, annual fish surveys funded by the Murray Darling Basin Authority’s Living Murray Program have found that while small-bodied fish populations are generally strong, populations of large-bodied fish such as the Murray cod are in decline. A number of factors may have contributed this decline, including poor water quality (such as the blackwater following the 2010 floods), inappropriate flows for fish migration and spawning, insufficient food, lack of appropriate habitat and lack of connection between habitats due to water regulating structures.

In an effort to encourage Murray cod spawning and juvenile development, the North Central Catchment Management Authority (CMA) is managing a changed environmental flow along the Gunbower Creek. Kathryn Stanislawski, Project Officer with the North Central CMA explains, “The CMA received an environmental water allocation for delivery from July 2013 to June 2014. Together with Goulburn-Murray Water (G-MW) and fish ecologists we worked closely to manage flows during the critical spawning period in late 2013.

North Central CMA project officer, Amy Russell assisting with the collection of a light trap

“Particular effort from G-MW has resulted in a more stable flow rate at Cohuna Weir. The flow regime has been managed to support spawning activity, to enable Murray cod to protect nests and to enhance larval dispersal into productive areas amongst aquatic vegetation on the edges of the creek. Results from monitoring have confirmed that during October – December, Murray cod larvae have been found in three age cohorts, indicating there has been at least three successful spawning events throughout the managed environmental flow event,” Ms Stanislawski said.

“The first rounds of monitoring during October and November 2013 found relatively high numbers of Murray cod larvae downstream of Cohuna, confirming that Murray Cod were spawning in Gunbower Creek. Monitoring throughout December 2013 continued to find cohorts of Murray cod larvae, rapidly advancing in size, having absorbed the remainder of their yolk sack and beginning to feed on very small prey,” Ms Stanislawski said.

The larvae are the size of a matchstick head and can be hard to see by the naked eye

Inundated aquatic vegetation in the riparian zone of Gunbower Creek

Dr Clayton Sharpe, who leads the project team and has been conducting the annual monitoring on Gunbower Creek since 2005, is confident this year’s Murray cod spawning events have been enhanced by the managed environmental flows.

“Gunbower Creek already has the habitat qualities to provide suitable spawning and nursery habitat for Murray cod. With appropriate flow management the Murray cod population in Gunbower Creek has high potential for recovery.

“A similar type of flow regime targeting spawning of large bodied fish is also being delivered in other areas of the Murray-Darling River system this year and together, through this research, we are forming a body of knowledge to improve the management of these iconic fish species for future generations,” Dr Sharpe explained.

Fish ecologist, Dr Ivor Stuart is providing advice about the flow regime and the larval monitoring project’s design. Dr Stuart said, “Murray Cod can produce thousands of larvae from one successful spawning event, and although there is a high mortality of young fish, those that survive their first summer and winter have a good chance of surviving through to maturity.

“Accordingly, we will recommend that environmental flows continue to be delivered through Gunbower Creek in a pattern that targets within-channel flows, without major changes to normal irrigation flow delivery, to maximise Murray cod spawning and larval access to food resources and shelter from predation.”

Further monitoring will occur during early 2014 to assess movement patterns of fish through a tagging project that monitors the movement of fish in Gunbower Creek in response to optimal flows.